What is Universal Time (UT)?
UT refers to a time scale called "Coordinated
Universal Time" (UTC) which is the basis for the worldwide system of civil time,
and is therefore the international time standard. This time scale is determined
using highly precise atomic clocks around the world.

"Universal Time" is the current term for what was commonly referred to as Greenwich
Mean Time (GMT). Zero (0) hours UTC is midnight in Greenwich England, which
lies on the zero longitudinal meridian. (The standard of UT replaced GMT in
1926 because too many different definitions of GMT were in use.)
Universal time is based on a 24 hour clock, therefore, afternoon hours such
as 4 pm UTC are expressed as 16:00 UTC.

More about time zones
The establishment of time zones accounts
for the fact that for any given instance the Sun is rising in one place on the
Earth, is standing high in the sky at noon for another place, and is setting
for a third place. Considering these astronomical facts, it makes sense to use
different civil time scales at different places on the Earth.
And since a day is 24 hours long, the world may be split into 15 degree wide
longitudinal bands (360 degrees/24 hours) or time zones. Each band represents
one hour. Therefore the local time of a time zone usually differs by a whole
number of hours from Universal Time which coincides with the local time on the
zero longitude. The difference "local time" minus "Universal Time" is positive
(+) for time zones east of Greenwich and negative (-) for western time zones.

Some frequently mentioned time zones
are:
Western European Time ( = Universal Time, difference 0 hours)
Central European Time (+1 hour)
in the USA:
Atlantic Standard Time (-4 hours)
Eastern Standard Time (-5 hours, east coast)
Central Standard Time (-6 hours)
Mountain Standard Time (-7 hours)
Pacific Standard Time (-8 hours)
Moscow Time (+3 hours)
Tokyo Time (+9 hours)

Daylight saving time is decreed for entirely
political reasons and has no astronomical basis.